The business end of Bridget Holmes's broom would have been something to keep well upwind of. But the towering portrait showing this tool, and the woman whose job it was to scour out the royal chamber pots, was hung at a prince's elbow in his dining room.

"It is a remarkable picture," said Giles Waterfield, one of the curators behind the National Portrait Gallery's exhibition. "In one way it's a joke, a parody of all those martial portraits of dukes and generals. But the figure herself is treated with great dignity, very respectfully. The joke's not on her."

The painting, by John Riley, has been lent by the Queen for the first exhibition devoted to portraits of servants.

Dated 1686, the portrait includes an inscription noting that the subject, Mrs Holmes, was 96. Her career, as a "necessary woman", one of the lowliest positions in a great household, spanned four monarchies - that of Charles I and II, James II, who probably commissioned the portrait, and William and Mary. Until last week this depiction of Mrs Holmes was hanging in Windsor Castle's state apartments .

The joint curators of Below Stairs, which opens at the London gallery today and lasts until January 11, are Mr Waterfield and Anne French, who found portraits of servants by the score in museum stores and dim passageways of country houses.

The earliest face in the exhibition is Tom Derry, Anne of Denmark's grave-faced jester, painted in 1614. The most recent is that of the Holkham estate workers with their employer, the Earl of Leicester, painted 10 years ago.